Paranoid privacy nuts. Systems designed by these people never
become popular because paranoid people don't have any friends.
(Examples: ZKS, Freenet, GPG.)

Redundancy leeches. These people want to back up their files
(encrypted) to your computer for added redundancy. Unfortunately, you gain
nothing by doing this for them; there's no way to force a leech to
contribute space back to other leeches. So these tend to end up as for-profit
services. (Examples: AllMyData, Dropbox, S3.)

Sharers. These people have data they want to share with
other people. They benefit by giving you the data; you benefit by
receiving the data, and if you like it, you'll feel nice by sharing
it further. (Examples: Debian, Wikipedia, BitTorrent.)